A monopoly in the original sense is when a specific enterprise is the only supplier of a commodity. The commodity here would be computer game distribution, and although Valve with Steam was and still is the biggest part of the market, it obviously couldn't crush any would-be competitors with its powers, like monopolies tend to.The closest thing in this situation to a monopoly is Steam itself, and people are rushing to defend it largely (imho) because they have a ton of games in their Steam libraries and do not want another launcher to mess with.
On the other hand, they gave plenty back to gamers. All the middleware (it's easy to forget about this if you aren't in game development), SteamVR, lots of Linux support and help lately... Actually, speaking of which, I find it ironic how quickly people would be upset about whatever the Epic launcher might be doing, but are using Windows 10 with its far more intrusive data collection.
Meanwhile, from my perspective, Epic has a much worse-featured launcher, without any plans for Linux support, and the only way they seem to be able to gain any competitive advantage is to spend vast amounts of money to buy exclusivity from developers. It doesn't really look like they can compete on merit alone so far.
But hey, the markets aren't rational, and money talks.